The term “microfluidic” refers to a system or device having one or a network of chambers and/or channels, which have micro scale dimensions, e.g., having at least one cross sectional dimension in the range from about 0.1 μm to about 500 μm. The term “microfluidic” in the context of device, system, disc etc refers to the fact that liquid volumes in the μl-range are transported within the system. This means that there may or may not be liquid transport within a certain part of the system, for instance the microfluidic device/disc. The μl-range includes the nl-range as well as the picolitre range. Microfluidic substrates are often fabricated using photolithography, wet chemical etching, injection-molding, embossing, and other techniques similar to those employed in the semiconductor industry. The resulting devices can be used to perform a variety of sophisticated chemical and biological analytical techniques.
Microfluidic analytical systems have a number of advantages over conventional chemical or physical laboratory techniques. For example, microfluidic systems are particularly well adapted for analyzing small sample sizes, typically making use of samples on the order of nanolitre and even picolitre. The channel defining substrates may be produced at relatively low cost, and the channels can be arranged to perform numerous analytical operations, including mixing, dispensing, valving, reactions, detections, electrophoresis, and the like. The analytical capabilities of microfluidic systems are generally enhanced by increasing the number and complexity of network channels, reaction chambers, and the like.
Substantial advances have recently been made in the general areas of flow control and physical interactions between the samples and the supporting analytical structures.
Flow control management may make use of a variety of mechanisms, including the patterned application of voltage, current, or electrical power to the substrate (for example, to induce and/or control electrokinetic flow or electrophoretic separations). Alternatively, fluid flows may be induced mechanically through the application of differential pressure, acoustic energy, or the like. Selective heating, cooling, exposure to light or other radiation, or other inputs may be provided at selected locations distributed about the substrate to promote the desired chemical and/or biological interactions. Similarly, measurements of light or other emissions, electrical/electrochemical signals, and pH may be taken from the substrate to provide analytical results. As work has progressed in each of these areas, the channel size has gradually decreased while the channel network has increased in complexity, significantly enhancing the overall capabilities of microfluidic systems.
The microfluidics technologies/devices are capable of controlling and transferring tiny quantities of liquids to allow biological assays to be integrated and accomplished on a small scale.
Microfluidics is the miniaturization of biological separation and assay techniques to such a degree that multiple “experiments” can be accomplished on a “chip” small enough to fit in the palm of your hand. Tiny quantities of solvent, sample, and reagents are steered through narrow channels on the chip, where they are mixed and analyzed by such techniques as electrophoresis, fluorescence detection, immunoassay, or indeed almost any classical laboratory method.
Today a number of products varying in many respects are available. Laboratory chips may be made from plastic, glass, quartz or even silicon. The fluid may be driven by centrifugal forces, mechanical pressure or vacuum pumps, by inertia, or by one of several electrical methods; fluid flow can be diverted around the chip by mechanical valves, surface tension, voltage gradients, or even electromagnetic forces.
In the technique of using centrifugal forces to drive the fluid a disc that can be spun is used. Some discs have been of the same physical format as conventional CDs. Samples are placed near the center of the disc and centrifugal forces, created as the disc rotates, push them out through channels cut into the plastic, circumventing the need to design sophisticated electrokinetic or mechanical pumping structures.
As will become evident in the forth-coming description the present invention is in particular applicable to (but not limited to) micro-analysis systems that are based on micro-channels formed in a rotatable, usually plastic, disc, often called a “lab on a chip”. Such discs can be used to perform analysis and separation on small quantities of fluids.
In order to reduce costs, it is desirable that discs are not limited for use with just one type of reagent or fluid, but should be able to work with a variety of fluids. However, scientists, laboratory staff, etc in a laboratory handles a lot of different chemical samples. To be able to perform and run different chemical tests for a various number of samples, the operators of the microfluidic system will have to handle a great number of different microfluidic devices. A common problem is to identify a certain microfluidic device, e.g. within a series of identical microfluidic devices used within a series of experiments.
Said identification problem may be solved in different ways, e.g. by using different kind of labels attached to the microfluidic device structure. One kind of labels store identity information in an electronic memory chip. The chip is readable, but may also be writable and/or erasable. To get access to the memory chip content for reading, writing, or erasing purposes such means, equipment and program software have to be added to the analysis system instrument. All extra equipment and program software increase the total cost for the system and also the system instrument. The adding of an electronic label to the microfluidic device adds an extra cost to the total cost and increases the manufacturing complexity of the microfluidic device. Suggested solutions of the identification problem are therefore not considered to be satisfactory from an economical point of view.
Furthermore, it is often desirable during the preparation of samples that the disc permits the user to dispense accurate volumes of any desired combination of fluids or samples without modifying the disc. A microanalysis device for fluids provided in a rotatable disc is described e.g. in WO-01/46465. A liquid transfer station has a robot that transfer at least one sample or any other predetermined liquid aliquot at a time from the sample and reagent station to a microfluidic device, for instance in the form of a disc that can be spun. The station has means for transfer of liquid samples, and other liquids, for instance a number of injection needles connected to syringe pumps or a number of solid pins may be used for the transfer of samples. Said needles and pins may be configured in different numbers of rows and columns having different distance between the tips in both directions. Another alternative is the microdispensor described in WO 9701085.
The needles or pins have to be exactly maneuvered to an appropriate inlet of each channel. The microfluidic devices, e.g. microfluidic discs, may be designed in different ways and each microfluidic device differs individually due to the manufacturing process. A home position mark is preferably placed in an outer circumferential zone outside the detection areas or in some other position, which can be detected with high accuracy. The position coordinates of each specific spot of the surface of the disc is given as the angular position relative to the home position mark and as the radial position relative to the circumference or axis of symmetry or relative to any other arbitrary fixed position on the device. The process or method for finding the home position mark and determining is called for “Homing the device (disc)” and the “Homing” process. Known homing processes involves the scanning of the disc for finding an edge of the home position mark.
In WO 03087779 there is described a known homing process. This process involves a check if a mark is a home position mark or not. One home position mark on a rotating disc placed on a disc holder of a microfluidic instrument is detected by a home position mark detector when a home mark is passing. Said known method comprises following phases:                a disc scanning phase, wherein a disc is scanned for mark edges;        a home mark identifying phase, wherein the false home marks are rejected and the correct home phase is identified;        a home position determining phase, wherein the home position is determined by use of the exactly determined edges of the home position mark.        
Said method has increased the possibility to locate the exact home position. However, some times the detected edge is a false edge of a home position mark. If a defect or pollution is present in the close vicinity of an edge of a home position mark, said defect or pollution may influence the determination of the exact location of the home position. Even a very small deviation from the true home position may ruin a whole experiment run as the injection needles are not placed properly in the inlets of the micro-channels. At occurrence of home position displacement, the aliquots and wash-liquids will be dispensed and wasted on the surface of microfluidic device.
Another problem with the use of only one home position mark is that the microfluidic device may be discarded if one of the edges of the home position mark is a defect edge. The instrument system will not be able to identify the home mark and the microfluidic device will be rejected by the system.
Another problem with the known home position mark is that the home position detector is capable of detecting the home position mark even though the microfluidic device is placed up-side down. The instrument system will not react for this kind of fatal mistake resulting in warning or alarm and/or a stop of the experiment run as the home position mark is locatable. All aliquots and wash-liquids will be dispensed and wasted on the surface of the bottom of the microfluidic device.